LONDON—Russian natural-gas giant OAO Gazprom won't stop pumping gas to its European customers even if Ukraine doesn't pay its arrears—-expected to reach $3.5 billion next month—but it can't guarantee those supplies will reach its intended customers, the company's Deputy Chief Executive Alexander Medvedev said Friday.

Russian state-controlled Gazprom supplies about 30% of Europe's gas needs, around half of which travels through pipes that cross Ukraine. Any disruption to those flows could have a significant impact on...